Alumina feed control

ABSTRACT

A method of controlling the feeding of alumina to a reduction cell and apparatus therefor. The method comprises the steps of producing a gradient variation in the supply current fed to the cells and obtaining at least one measurement of the voltage and the current across the cell for a first value of the current gradient and obtaining at least a second measurement of the voltage and current across the cell for a different value of the current gradient. An indication of the alumina concentration is derived from the first and second current and voltage current measurements. An amount of alumina which is related to the determined value of alumina concentration and which will restore the desired operating alumina concentration is then added to the cell.

United States Patent [72] Inventor Donald R. Bristol Orinda, Calif.

[21 Appl. No. 707,747

[22] Filed Feb. 23, 1968 [45 Patented Dec. 21, 1971 [73] Assignee Kaiser Aluminum & Chemical Corporation Oakland, Calif. I 1

[54] ALUMINA FEED CONTROL POWER SUPPLY CRUST IIIAKIR 3,455,795 7/1969 Boulanger et al 3,47 I ,390 10/1969 Kibby et al FOREIGN PATENTS l2l,938 9/1958 U.S.S.R.

ABSTRACT: A method of controlling the feeding of alumina to a reduction cell and apparatus therefor. The method comprises the steps of producing a gradient variation in the supply current fed to the cells and obtaining at least one measurement of the voltage and the current across the cell for a first value of the current gradient and obtaining at least a second measurement of the voltage and current across the cell for a different value of the current gradient. An indication of the alumina concentration is derived from the first and second current and voltage current measurements. An amount of alumina-which is related to the determined value of alumina concentration and which will restore the desired operating alumina concentration is then added to the cell.

CELLS I -(LAST-I) PATENTED 0:021 m2 SHEET 2 BF 2 IN' VENTOR DONALD R. BRISTOL F93 jwu EUXQURQ LWDRU EMSO Milan WWW ATTORNEY ALUMINA FEED CONTROL BACKGROUND OF THE INVENTION The metal aluminum is extracted from aluminum-bearing compounds such as alumina (A1 by electrolysis from a molten cell bath or electrolyte. In the production of aluminum by the conventional electrolytic process, commonly referred to as the Hall-Heroult process, the electrolytic cell comprises in general a steel shell having disposed therein a carbon lining. The bottomof the carbon lining, together with a layer of electrolytically produced molten aluminum which collects thereon during operation, serves as the cathode. One or more consumable carbon electrodes is disposed from the top of the cell and is immersed at its lower extremity into a layer of molten electrolyte which is disposed in the cell. In operation, the electrolyte or bath which is a mixture of alumina ad cryolite is charged to the cell and an electric current is passed through the cell from the anode to the cathode via the layer of molten electrolyte. The alumina is dissociated by the current so that aluminum is deposited on the liquid aluminum cathode and oxygen is liberated at the carbon anode, forming carbon monoxide and carbon dioxide gas. A crust of solidified electrolyte and alumina forms on the surface of the bath, and this is usually covered over with additional alumina.

In the conventional electrolytic process, use has been made of two types of electrolytic cells, namely that commonly referred to as a prebake" cell, and that commonly referred to as a Soderberg" cell. With either cell, the reduction process involves precisely the same chemical reactions. The principal difference is one of structure. ln the prebake cell, the carbon anodes are prebaked before being installed in the cell, while in the Soderberg, or self-baking anode cell, the anode is baked in situ, i.e., is baked during operation of the electrolytic cell, thereby utilizing part of the heat generated by the reduction process. The instant invention is applicable to either cell.

A typical aluminum electrolytic bath used in commercial installations might have the following composition:

l-l0% alumina 0-l0% aluminum trifluoride 5-l2% calcium fluoride 80-90% cryolite As the electrolysis continues, alumina is consumed in direct proportion to the metal production. As the alumina concentration in the electrolyte is reduced, a point isreached where a troublesome phenomenon known as anode effect" occurs. During an anode effect, the voltage drop across the cell can increase, for example, from around 4 volts to as much as 40 volts and even higher. This effect is generally attributed to too low a concentration of alumina in the reduction cell bath or electrolyte. The actual concentration of alumina in the electrolyte at which this effect occurs seems to depend upon the temperature, the composition of the electrolyte, the anodecathode distance and the anode current density. The occurrence of the anode effect is the signal for the addition of more alumina. The attendant does this by breaking the frozen crust on top of which he has previously distributed a'layer of alumina. The addition of the alumina, as well as a vigorous stirring of the electrolyte, causes the anode effect to disappear, after which the electrolysis continues its normal course until the next anode effect occurs.

There are several disadvantageous results of an anode effect such that minimizing or substantially eliminating their occurrence is desirable. The cell overheats rapidly during an anode effect and this overheating causes very rapid consumption of the anode and excessive consumption of electrolyte by volatilization and results in a lowered yield of product. A very important undesirable result of the anode effect is a large unproductive power consumption.

One important approach to the problem of detecting the oncoming of an anode effect is outlined in an article by Mc- Mahon, T. K. and Dirth, G. P., Computer Control of Aluminum Reduction Cells,"J0umal of MetaIsNol. 18, No. 3, pp.

317-3 l9, March, 1966. The approach outlined therein involves measuring the total voltage drop across the cell, from collector bar of one cell, to collector bar of the next, and noting when this voltage starts to rise rapidly. This is the oncoming of an anode effect and the anode effect termination procedure is then initiated and hopefully the occurrence of an anode effect is avoided.

This procedure, theoretically, is useful in avoiding anode effects, but because the total resistance curve as measured by the total voltage drop across the cell changes very little until theoncoming of an anode effect, the exact alumina concentration in the cell electrolyte at any given time cannot be determined by this method. It is considered desirable to maintain a constant alumina concentration in the cell and hence a constant cell operation.

In order to maintain a substantially constant alumina concentration, one must measure in some way the concentration of the alumina in the bath or electrolyte, or the feed rate in order to control the feed rate. The prior art describes many methods which have been utilized in the past to determine the alumina concentration in the electrolyte or bath. These methodsinvolve chemical analysis for the alumina concentration, either by pyrotitration techniques, caustic leach methods, gravimetric methods of analysis, volumetric methods of analysis, or by means of electrical conductivity measurements. Examination of physical properties of the electrolyte has also been utilized to some extent. The appearance of the electrolyte, both molten and solidified, has been compared with known samples; crystalline phases have been examined by microscope and X-ray diffraction as well as in other ways. None of these prior art methods has been truly satisfactory. Those methods with high accuracy and reliability take too long to yield the desired information. Techniques which produce an answer more rapidly tend to be rather inaccurate. The present invention was developed against this background in the art.

SUMMARY OF THE INVENTION This invention relates to a method and apparatus for controlling the feed of alumina to a reduction cell. The feed is controlled for two purposes. One is to avoid anode effects and the other is to maintain a substantially constant alumina concentration in the cell which results in improved current efficiency of the reduction process. A gradient variation in the supply current fed to the cells is produced. This may be done in divers ways. One method is to program an anode effect into a cell or into alternate cells in the system. This will produce a gradient variation in the supply current fed to the cells. Another way of producing the gradient variation is through the utilization of a stepping transformer. Yet another method might be through the utilization of a saturable reactor connected in series with the power supply to the cells.

At least one measurement of the voltage and the current across the cell is obtained for a first value of the current gradient. At least one more measurement of the voltage and current across the cell for a different value of the current gradient is obtained. An indication of the alumina concentration of the cell bath is then derived from the first and second current and voltage current measurements. The indication may be derived by determining the back electromotive force (back EMF) in the cell or by determining the cell bath resistance from the current and voltage current measurements. If the back electromotive force is used, it is determined from the measurements according to the relationship:

where V is the cell voltage in volts, I is the cell supply current in amperes, B is the cell bath resistance or change in voltage per change in current as determined by the first and second current and voltage current measurements, and A is the back electromotive force.

If the cell bath resistance is used, it is determined from the measurements according to the relationships:

B=AvlAi (ii) where Av/Ai is the change in voltage per change in current as determined by the first and second current and voltage current measurements, and B is the cell bath resistance.

The cell may be electrically connected to a plurality of other reduction cells, all in series, so that an indication of the alumina concentration in each cell can be derived during a single gradient variation in the supply current.

It has been found that the alumina concentration where the optimum efficiency of operation of the reduction cells as measured by the current efficiency of the cells is attained seems to be quite close to the level at which an anode effect occurs. When an anode effect occurs, the cell reaction seems to change from a decomposition of alumina to a decomposition of the aluminum fluoride in the bath. Thus, the alumina concentration should be maintained by this process and apparatus above the value at which the decomposition voltage for aluminum fluoride is attained but at the same time lower than about 4 percent by weight. The precise value at which the decomposition voltage for aluminum fluoride would be obtained will vary somewhat from cell to cell but should always be below about 4 percent by weight. Desirably, the alumina concentration should be maintained below about 3 percent by weight. More specifically, in the normal American reduction cell the alumina concentration should be maintained from about 2 percent to about 3 percent by weight.

An important function of the process and apparatus of this invention is to feed the alumina substantially continuously to the cell so as to minimize variations in the concentration of alumina in the cell and improve the current efficiency of the operation (even though fluoride consumption will increase somewhat).

Should a variation in the anode'cathode distance (ACD) to the desired operating level be necessary, this should be done before producing the gradient variation in the supply current so that the cell resistance curve can stabilize after the anodecathode distance adjustment is made and before the gradient variation in the supply current is produced. From this, it also follows that the anode-cathode distance should be maintained constant while the gradient variation in the supply current is produced.

The apparatus for controlling the feeding of alumina to the reduction cell comprises means for producing a gradient variation in the supply current. Various suitable means have been discussed supra. Voltage measuring means are connected across the cell. The voltage measuring means and the current measuring means are operably related so that a measurement of the voltage and the current across the cell may be obtained for a first value of the current gradient and also for a second value of the current gradient. Suitable means are provided for deriving an indication of the alumina concentration of the cell bath from the first and second current and voltage current measurements. This might simply be a chart or nomograph relating the alumina concentration to either the back electromotive force or the cell bath resistance or desirably a computer can be utilized for performing the necessary computations. Finally, anode feeding means responsive to the indication of alumina concentration of the cell bath are provided for adding an amount of alumina to the cell and restoring the desired operating alumina concentration.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a typical graph showing the relationships between the percent alumina in the cell bath or electrolyte and the cell bath resistance, the back electromotive force, and the total cell resistance (measured from collector bar of one cell to the collector bar of the next cell).

FIG. 2 is a typical graph showing representative variations in voltage and current measurements across a cell for different values ofa gradient variation in the supply current to the cell.

FIG. 3 is a schematic showing ofa computer controlled potline.

DETAILED DESCRIPTION The article by McMahon and Dirth mentioned above discusses a control scheme for reduction across the cell. The authors thought that there were three short term variables in this resistance; the anode-cathode distance, specific bath resistivity, and resistance of the gas film surrounding the anode. The authors felt that both the bath resistance and the film resistance were affected by the alumina concentration. However, in reality, assuming constant anode-cathode distance, the cell resistance is then determined by what may properly be described as two factors. One of these is the bath or electrolyte resistance and the other is best characterized as the back electromotive force.

The back electromotive force is not properly a pure resistance in teat it involves the decomposition potential for the reaction of the alumina and the cathode gas film polarization and the anode gas film polarization. The McMahon article assumed that the decomposition potential was a constant. In reality it is not. It has been discovered that the back electromotive force builds up constantly as the alumina concentration in the electrolyte decreases. This is important in that McMahon et al. thought that it would only rise rapidly at the onset of anode effect when the gas film polarization builds up rapidly. Actually, as shown by curves (B), (A), FIG. I, the changes in bath resistance and back electromotive force are roughly inversely proportional to one another. The back electromotive force will build up as the alumina concentration decreases until the voltage reaches a value for the carbon tetrafluoride reaction (i.e., the decomposition of aluminum fluoride to react with the carbon and form carbon tetrafluoride) to occur, rather than the alumina reaction. At this point, the film resistance builds up rapidly as shown by McMahon. Because of this, measuring the total cell resistance is useful as an anode effect prewarning device. However, the total resistance curve (see curve total R, FIG. I) is relatively flat until just before an anode effect occurs when the gas film polarization causes the resistance to rise rapidly. Moreover, electrical noise or disturbances in the circuitry can mask this change in the total resistance curve until the end point has been reached.

The instant invention is based upon the concept that if the back electromotive force and the cell bath or electrolyte resistance could be separately determined, the alumina concentration in the cell bath electrolyte could be determined from the value of either one of them and hence could be known at all times. Ifthe alumina concentration is known, then one will know when to feed the cell additional alumina to avoid an anode effect or to maintain a desired operating alumina concentration in the electrolyte.

Other methods for measuring the alumina concentration in the electrolyte are known. These are all either too inaccurate or too slow for the type of control contemplated. For example, wet chemical analysis requires approximately 30 minutes to perform. Knowing what the concentration of alumina in the bath was 30 minutes ago is not very helpful for a precise concentration control or for avoiding an anode effect when one desires to operate the cell at a feed concentration close to the level at which an anode effect occurs. Moreover, a sample taken may not be representative of the true concentration in the bath. This could, for example, occur if the sample is taken from a relatively quiescent zone of the cell. More rapid analytical techniques than wet chemical analyses generally sacrifice accuracy for speed. What is needed is an accurate analytical technique that will keep up with the process, in a time scale of seconds or less, i.e., a real time analysis.

The instant invention solves this problem and enables one to have real time knowledge of the alumina concentration of the cell during operation.

The voltage drop across the cell may properly be represented by the equation:

where B equals the cell bath resistance, A equals the electromotive force, and V equals the cell voltage in volts measured from collector bar of one cell to collector bar of the next cell in series. Typical curves for the back electromotive force and cell resistance at a given and constant anode-cathode distance and the relationship to the percent alumina in the electrolyte are shown in FIG. 1. For comparison purposes, the total resistance curve (total R) is also shown.

One determines the values for the back electromotive force and cell bath resistance by producing a gradient variation in the supply current to the cells. Various methods of producing this variation have been mentioned previously. It has also been mentioned previously that the anode-cathode distance is held constant while this gradient variation is being produced and one waits until the cell resistance curve has stabilized after an anode-cathode distance adjustment has been made before producing the gradient variation in the supply current. Measurements of the voltage and current across the cell for different values of the current gradient are taken. These may be plotted on a graph as shown in FIG. 2. The slope of the line through them is equal to B. Stated in a different way:

B=Av/Ai. (ii) where Av/Ai is the change in voltage per change in current as determined by these measurements. Knowing the value of the cell bath resistance, one can then calculate the back electromotive force by the relationship discussed above:

Either of these can then be related directly to the percent alumina in the electrolyte and the alumina concentration adjusted as desired.

As indicated above, this may be done manually, but the process is particularly adaptable to computerization. There are many process control computers available which can be used for this. One such computer is that known in the trade as a GE/PAC 4050 I. This computer is specifically designed for process control and real time operation and has a core size of 12,000 24 binary bit words. It has a memory cycle speed of microseconds with no bulk memory. The inputs to the computer comprise three groups of 20 digital inputs, groups of l6 analog inputs, one paper tape reader with a capability of 100 characters per second, and one operator console. The outputs from the computer comprise eight groups of 16 digital outputs, one paper tape punch with a capacity of I20 characters per second, and two remote console output typewriters.

Thus, the computer has a real time monitor program having a plurality of functional programs, certain of which control the process and others of which are service and recording programs. A discussion of these anciliary programs is not necessary to understand the instant invention.

Using the computer, the two variables can be solved for simultaneously rather than first determining the cell bath resistance. In practice, it has been found that the cell bath resistance curve seems to be effected by gas film on the bottom surface of the anode. The extent to which this occurs seems dependent upon the topography of the bottom surface of the anode. Once this effect has been determined and the cell bath resistance curve established, it is then possible to relate that curve to the percent alumina. However, it has been found that the back electromotive force curve varies little from cell to cell at constant anode-cathode distance (ACD). The shape of the curve does not change for different ACD values, only the numerical value of a given point on the curve. Once the ACD has been established, it is then possible to fix the location of the curve. Therefore, in certain applications it may be more practical to operate with the back electromotive force value rather than the cell bath resistance value.

As can be seen from FIG. I, the alumina concentration in the bath cannot be very readily determined by total resistance measurements (total R curve) until one is well into the anode effect prewarning situation. It has recently been found that the greatest current efficiency in a reduction cell seems to occur when the cell is operating on what may be called a lean feed." A lean feed may be defined as a quantity of alumina in a bath just sufficient to prevent the cell from going on anode effect. While the operating characteristics of the cell such as temperature, depth of electrolyte, anode-cathode space, etc., may effect this value, it can be expressed in terms of decomposition potentials. Sufficient alumina is maintained in the cell to prevent the back electromotive force from building up until the voltage reaches the value for the decomposition of the aluminum fluoride to form carbon tetrafluoride. Thus, the alumina concentration must be maintained above the value at which the decomposition voltage for aluminum fluoride is attained but close to it. A useful upper limit on concentration would seem to be about 4 percent by weight. A preferred upper limit would be about 3 percent weight and perhaps the optimum concentration would be from about 2 percent to about 3 percent by weight.

As can readily be seen from FIG. 1, it is not possible to maintain a very fine control on the alumina concentration if one seeks to utilize the total resistance curve. The total resistance curve is not sufficiently sensitive to changes in alumina concentration until quite close to the anode effect. However, the back electromotive force or the cell bath resistance vary significantly with the alumina concentration, and the alumina concentration at any given time can be accurately deter mined from these measurements. Accordingly, the process and apparatus of the instant invention make it possible for the first time to operate successfully on lean feed.

With reference now to FIG. 3 which shows a computer controlled potline in schematic form, it may be seen that the cell comprises first a metal shell 10, generally steel, within which is disposed in the usual manner an insulating layer 12 which can be any desired material, e.g., alumina, bauxite, clay, aluminum silicate brick, etc. Within the insulating layer 12 is disposed cell lining 14 which can be of any desired material, e.g., carbon, alumina, fused alumina, silicon carbide, silicon nitrate, bonded silicon carbide, or other desired materials. Most commonly, the lining is made up of a plurality of carbon blocks or is a rammed carbon mixture or a combination of a rammed carbon mixture for the bottom of the lining with side and end with side and end walls constructed of carbon. Alternatively, the side and end walls can be constructed of blocks of silicon carbide or other suitable refractory. The lining l4 defines a chamber which contains a pool of molten aluminum l6 and a body of molten electrolyte or bath 18 as described.

Suspended from above the electrolyte and partially immersed therein are anodes 20 of the conventional carbon type and shown here as a prebaked anode. The molten electrolyte I8 is covered by a crust 22 which consists essentially of frozen electrolyte constituents and additional alumina. As alumina is consumed in the electrolyte 18, the frozen crust is broken by a suitable crust breaker, not shown, and more alumina fed into the electrolyte by the opening of ore valve 24 which causes the alumina to be fed from the feed hopper 26. The anode is connected by anode bus bar 28 to the positive pole of a source of supply of electrolyzing current. For purposes of completing the electric circuit, use is made of cathodic current conducting elements or collector bars 30. The collector bars 30 extend through suitable openings provided in the metal shell and insulation layer with the inner ends thereof projecting into the cell lining, The outer ends of the element are connected by suitable means to the other side of the supply line.

As shown in FIG. 3, the cells are connected in series over a surge device 32 and an ammeter (I) 34 to a suitable power supply, one side of the supply being fed to the anode system of the first cell and the cathode of the first cell being in turn connected to the anode of the second cell, etc., the cathode of the last cell being connected to the other side of the supply line. The cells also include suitable means for raising and lowering anodes 20 such as an air motor or solonoid valve 36, a suitable crust breaker device (not shown) for each cell and the alumina ore drop previously discussed with the ore valves being operated in a suitable manner such as air operating through solenoid valves. A suitable volt meter 38 (El-E last) is connected between the anode and cathode of an associated cell to provide an indication of the voltage drop across the cell. A suitable computer 40 is connected into the system, as shown in the figure. Through an interface 42 the computer has a programmed read out connected to a surge device 32. Also operably connected to the computer 40 are operator's panels 44, tape punch 46, tape recorder 48 and typewriters 50.

The process scanning program may be generally described as one program ofa plurality of functional program used in the potline monitoring and control. In the process scanning program, which in one embodiment is programmed to occur at predetermined time intervals, the computer either programs a line surge by means of surge device 32 through interface 42 or programs an anode effect (i.e., by starving a selected pot, different pots being selected for this purpose in different read outs, and means being provided for sensing the anode effect so that the computer can shift to the scanning program) or when an anode effect occurs naturally and during the line surge selectively examines the pot line voltage of each of the cells as provided by the volt meters 38 (El-E last) connected across each of the pots on the line. Simultaneously, the computer reads the line amperage as provided by the ammeter 34 which is connected in one side of the line. The back EMF is then calculated by the computer 40 in accordance with the formulae previously described in the specification and stored in the memory. If the computer, in its scan of the read outs, detects that the back EMF is approaching a predetermined threshold (it being recalled that the back EMF curve is inversely proportional to the alumina curve and that the percentage of alumina is preferably maintained as low as possible, e.g., at from about 2 percent to about 3 percent by weight), a special sub program is entered immediately and control of the particular pot which is detected to be approaching the threshold is affected by signals from the computer over the crust breaker conductor to the detected cell to break the crust and signals over the alumina ore drop" conductors to the ore valve 24 in the detected cell to increase the alumina concentration in the molten bath. The electrolysis in the cell then continues its normal course until the cell is once more detected as approaching the predetermined threshold.

As indicated previously, the back EMF in each cell is basically determined by producing a gradient variation in the supply current fed to the cells, obtaining a measurement of the voltage and the current across the cell at different values of the current gradient, and calculating the back EMF on the basis of such information, and then adjusting the alumina feed to the cell. Such current gradient may be provided in several ways. In one way, the computer may be programmed to control the surge device shown in the drawing (i.e., a stepping transformer, a saturable reactor connector in series with the power supply or the like, to produce a surge in the line current). During the programmed read out, readings of voltage and amperage across each of the pots are fed to the computer at successive intervals on the current gradient.

In yet other installations, a heavy current drawing ap paratus, such as a cold rolling mill, may be connected to the same power supply and current surges in the line may be sensed by appropriate sensing means and fed to the computer. The computer, in turn, immediately enters into the process scanning program to obtain the desired read out.

In yet another mode of operation, an anode effect may be programmed into a cell or alternate cells in the system. That is, a computer may select a cell for an anode effect and in such instance will merely reduce the alumina feed into the cell until such time as the alumina concentration reduces itself to the point where the anode effect occurs. As a result, a current gradient will occur in the line, and with read out of the voltage and amperage across each of the pots in the line during such gradient, the calculation of the back EMF in each of the cells on the line is effected by the computer. A different cell is preferably programmed for each successive read out when such method is used. A naturally occurring anode effect can also be utilized for this purpose.

Tests were run in a conventional prebake anode cell such as shown more or less schematically in FIG. 3 containing eight prebaked anodes. The dimensions of the anodes were 20% inches X2095 inches l5 inches. The cell, during the test period, was operated at 25,000 amps.

In one test, the cell was operated at a voltage drop of about 5.1 volts. An average anode-cathode distance of 2.0 inches was maintained during the test. The cell averaged 7 inches of electrolyte and 4-74 inches of metal pad at the bottom. The cell bath temperature varied from approximately 980 to l,0O0 C. The measurements were made by producing a gradient variation in the supply current fed to the cells. Voltage and current measurements were made with a voltmeter and an ammeter, respectively. Measurements of the voltage and current across the cell for different values of the current gradient were made and the back electromotive force and cell bath resistance calculated by the procedure discussed above. At the same time, representative samples of electrolyte were taken and analyzed for alumina concentration by the conventional wet chemical analysis.

Table I gives the results of this test. The table shows the variation of back electromotive force (EMF) with varying alumina concentration and also shows the cell bath resistance values at the different alumina concentrations (Alumina added between measurements 10 and II).

Table II shows the results of another test run on the same cell but with different anodes of the same size. Here the cell was operated at a normal voltage drop of 5.08 volts and at an average anode-cathode distance of L inches. The electrolyte temperature averaged approximately 990 C. during the test. The depth of the molten metal pad averaged 5 inches and the bath depth averaged 6 inches during the test. This test illustrates two significant relations in comparison with table I. The first relationship is the dependency of the back electromotive force curve values upon the anode-cathode distance. It further illustrates that the curve shape is reproducible and only its position shifts with anode-cathode distance. The second significant relationship shows that the cell bath resistance values are significantly different. This is not accounted for solely by the change in the anode-cathode distance. As stated earlier, it is believed that this is due to a differing topography on the lower surfaces of the anodes in this test resulting in a different gas film build up. This might be due to some dependency of current density on the anode topography.

(Alumina added between measurements 7 and 8.)

These test illustrate the utility of the instant invention. As shown therein, the alumina concentration in the electrolyte can be determined at any time by following the procedure disclosed herein. Knowing the alumina concentration at any given time, it is possible to adjust the concentration as necessary to maintain it at a constant value. With constant measuring or with rapid accurate measurements of the alumina concentration possible, it is also possible to continuously feed a small quantity of alumina to the cell and thus maintain the operating alumina concentration even more closely to a desired constant value.

While there has been shown and described hereinabove possible embodiments of this invention, it is to be understood that the invention is not limited thereto and that various changes alterations and modifications can be made thereto without departing from the spirit and scope thereof as defined in the appended claims wherein:

1. In the production of aluminum by electrolytic reduction wherein the electrical current is supplied to a plurality of reduction cells connected in electrical series, the total line current and voltage drops across the individual reduction cells are monitored and the values used to determine the back EMF of the cells, and when the back EMF indicates that the alumina concentration in a cell is outside the desired limits, the alumina feed rate to said cell is adjusted to provide alumina in the desired concentration range, the improvement comprismg:

a. effecting a change in the electrical current to the cells by inducing an anode effect in one of the cells by reducing the alumina fed to said one cell;

b. determining by digital computer means the back EMF of each of the cells by solving the simultaneous equations for A where A is the back EMF, V, and I, are the cell voltage and line current, respectively, prior to the anode effect, and l, and I, are the cell voltage and line current, respectively, during the anode effect.

2. The method of claim 1 wherein different cells are selected for reduced feeding of alumina to induce an anode effect in successive program periods.

3. The method of claim 1 wherein the alumina concentration in the cells is maintained between about 2 and 4 percent by weight.

4. In the production of the aluminum by electrolytic reduction wherein an electrical current is supplied to a plurality of reduction cells connected in electrical series, a system for maintaining the alumina concentration in the reduction cells within predetermined limits comprising:

a. means for feeding alumina to each of the cells;

b. voltage measuring means connected across each of the cells;

c. current measuring means connected across the current flow to the cells;

d. a digital computer means programmed to i. select a particular cell to receive a reduced feeding of alumina to induce to induce an anode effect.

ii. reduce the feeding of alumina to said selected cell to induce an anode effect whereby current to said cells is changed,

iii. calculate the back EMF of each cell from the values of voltage and current prior to and during said anode effect by solving the simultaneous equations for A where A is the back EMF, V, and l, are the cell voltage and line current, respectively, prior to the anode effect and V and I, are the cell voltage and line current, respectively, during the anode effect,

iv. determine the alumina concentration in each of the cells by comparing the back EMF value with a predetermined tabulation of the relationship between back EMF and alumina concentration,

v. generate a signal for each cell which is utilized to adjust the feeding rate of alumina to each of the cells so as to maintain the alumina concentration therein within a predetermined range;

e. means responsive to said signals, to control the alumina feeding means.

5. The system of claim 4 wherein said digital computing means is programed to select different cells for reduced feeding rates during successive program intervals so that the induced anode effects are distributed among the cells electrically connected in series.

6. The system of claim 4 wherein the digital computing means is programmed to maintain the alumina concentration in each of the cells between about 2 and 4 percent by weight. 

2. The method of claim 1 wherein different cells are selected for reduced feeding of alumina to induce an anode effect in successive program periods.
 3. The method of claim 1 wherein the alumina concentration in the cells is maintained between about 2 and 4 percent by weight.
 4. In the production of the aluminum by electrolytic reduction wherein an electrical current is supplied to a plurality of reduction cells connected in electrical series, a system for maintaining the alumina concentration in the reduction cells within predetermined limits comprising: a. means for feeding alumina to each of the cells; b. voltage measuring means connected across each of the cells; c. current measuring means connected across the current flow to the cells; d. a digital computer means programmed to i. select a particular cell to receive a reduced feeding of alumina to induce to induce an anode effect. ii. reduce the feeding of alumina to said selected cell to induce an anode effect whereby current to said cells is changed, iii. calculate the back EMF of each cell from the values of voltage and current prior to and during said anode effect by solving the simultaneous equations V1 A+ AI1 V2 A+ BI2 for A where A is the back EMF, V1 and I1 are the cell voltage and line current, respectively, prior to the anode effect and V2 and I2 are the cell voltage and line current, respectively, during the anode effect, iv. determine the alumina concentration in each of the cells by comparing the back EMF value with a predetermined tabulation of the relationship between back EMF and alumina concentration, v. generate a signal for each cell which is utilized to adjust the feeding rate of alumina to each of the cells so as to maintain the alumina concentration therein within a predetermined range; e. means responsive to said signals, to control the alumina feeding means.
 5. The system of claim 4 wherein said digital computing means is programed to select different cells for reduced feeding rates during successive program intervals so that the induced anode effects are distributed among the cells electrically connected in series.
 6. The system of claim 4 wherein the digital computing means is programmed to maintain the alumina concentration in each of the cells between about 2 and 4 percent by weight. 